Abstract Our studies have demonstrated that developmental exposure to perfluorinated alkylate substances (PFAS) are associated with attenuated antibody responses to routine childhood vaccines. Our most recent findings suggest that breastfeeding can transfer PFAS to the infant and lead to substantial elevations of serum-PFAS concentrations, with possible adverse implications for immune system development. As blood samples from infancy were not available to us in previous studies, we modeled the concentrations of early-life serum-PFAS and showed that levels of early postnatal serum-PFAS are inversely associated with antibody concentrations measured at age 5 years, more so than serum concentrations measured at age 18 months or later. The present proposal will obtain blood samples in infancy and maternal milk for analysis of PFAS to improve the modeling of profiles of serum-PFAS during infancy. This will allow testing of the hypothesis that early-life exposure to immunotoxic PFAS impair the development of the adaptive immune system and negatively affects the efficacy of routine childhood vaccines. The extended model of serum-PFAS will be used for estimation of exposures of PFAS in infancy among >1,000 children from previous cohorts, where serum samples from infancy were not collected. In addition to blood sampling at ages 3 and 12 months in the new cohort, we will use novel cell phone technology to allow mothers to record breastfeeding, occurrence of infectious disease, antibiotic treatment, and other relevant study parameters every two weeks. Our focus on vaccines will include toxoids (diphtheria and tetanus), as these de novo protein vaccines are known to be the most reliable clinical indicators of immune suppression. The duration of breastfeeding will be considered both as an exposure pathway and a moderator, as breastfeeding is considered to be advantageous for the child?s immune system development. The proposed study will be carried out at the Faroe Islands, where excellent conditions are available to recruit the birth cohort and to ensure a high participation rate and minimal socioeconomic confounding. Exposures of PFAS mainly originate from seafood and vary substantially, while average serum concentrations are similar to U.S. levels. A cohort size of 600 can be recruited within 16 months and will provide appropriate statistical power as one of the largest so far in the field. At 3 and 12 months of age, vaccine antibodies, thymus size, and advanced differential white cell counts will be assessed. Statistical data analysis will include multivariate analysis, assessment of mediation and/or modulation, structural equation modeling of combined exposure of exposure associations of PFAS with immune functions, and benchmark dose calculations. While research on immunotoxicology has traditionally focused on adverse effects in the mature organism, this proposal aims to characterize immunotoxic risks from developmental exposure to these priority pollutants at the most vulnerable developmental stages in early life.